During the formation of a semiconductor device, contacts, for example to the substrate or word line, are commonly formed. FIGS. 1-4 describe a method for simultaneously forming a portion of a transistor, contacts to a word line and a substrate.
In FIG. 1, a semiconductor wafer substrate 10 having a front and a back is layered with various materials. For example, a gate oxide layer 12, a polycrystalline silicon (poly) layer 14, a tungsten silicide layer 16, and an oxide separation layer 18 are formed on the front of the wafer, then a nitride layer 20A and 20B is formed over the entire wafer, including both the front (20A) and the back (20B), for example by thermally growing the layer. Some of the other materials (for example the gate oxide 12 and poly 14) may also be formed on the back of the wafer and removed by other processing steps. A patterned photoresist (resist) layer 22 is formed over the nitride layer 18 on the front of the wafer. The layers are etched using the resist layer 22 as a mask.
Next, a layer of spacer material 24, for example oxide or nitride, is formed over the front and back of the wafer to result in the structure of FIG. 2. A spacer etch can be performed at this point to form the spacers 30 as shown in FIG. 3, or the spacer etch can be formed after the back side nitride etch described later. The front of the wafer is protected, for example with a mask (not shown), and the material on the back of the wafer is removed, for example with a wet or, preferably, a dry etch. The material on the back of the wafer is removed to reduce the stress on the wafer caused by the properties of the nitride film. Wafer processing continues, for example to form a first layer of borophosphosilicate glass (BPSG) 32, a layer of tetraethyl orthosilicate (TEOS) 34, and a second layer of BPSG 36 as shown in FIG. 3. A resist layer 38 is patterned over the wafer, leaving exposed the areas of the wafer to which contacts are to be formed.
An etch is performed in an attempt to result in the structure of FIG. 4A. FIG. 4A shows a structure in which a contact 40 is made to the substrate 10 and a contact 42 is made to the layer of tungsten silicide 16. The layers 12-20 show a stack which forms a portion of a transistor (a word line stack). In a conventional process, stopping the etch at the substrate and at (or within) the layer of tungsten silicide (or some other layer) is difficult using a single etch, for example because the nitride etches slower than the BPSG. Typically, to achieve the tungsten silicide contact 42 a portion of the substrate will be removed as shown in FIG. 4B, which can produce an electrically undesirable cell. If the etch is stopped at the substrate, often contact will not be made to the tungsten silicide 16 but the contact will instead stop within the oxide 18 or nitride 20 as shown in FIG. 4C.
One method of solving this problem is to mask and etch the contact to the substrate, then mask and etch the contact to the tungsten silicide. This, however, adds an extra mask step which can create problems with alignment.
A process which forms a contact to the substrate and to another layer without adding an additional mask step is desirable.